1. Technical Field
The present disclosure relates to a tractor unit for conveying fanfold paper or other continuous paper having sprocket holes, to a conveyance device including the tractor unit, and to a printer having the conveyance device.
2. Related Art
Japanese Unexamined Patent Appl. Pub. JP-A-2006-8265 describes a conveyance device for conveying continuous paper. The conveyance device disclosed in JP-A-2006-8265 has a paper feed roller, and a tractor unit disposed on the upstream side of the paper feed roller in the conveyance direction of the continuous paper. The tractor unit in JP-A-2006-8265 has a first tractor, a second tractor, a support shaft, and a clamping mechanism. The first tractor has first tractor pins that can engage first sprocket holes formed along one side in the width direction of the continuous paper. The second tractor has second tractor pins that can engage second sprocket holes formed along the other side in the width direction of the continuous paper. The support shaft supports the first tractor and second tractor so that both tractors can move in a transverse direction perpendicular to the conveyance direction of the continuous paper. The clamping mechanism holds the second tractor in a fixed position on the support shaft.
Continuous paper is set in the conveyance device described in JP-A-2006-8265 by engaging the first sprocket holes in the continuous paper with the tractor pins of the first tractor set to a reference position, then sliding the second tractor along the support shaft to a position appropriate to the width of the continuous paper and engaging the second sprocket holes of the continuous paper on the tractor pins of the second tractor, and then clamping the second tractor to the support shaft.
The conveyance device described in JP-A-2006-8265 conveys continuous paper that has already been printed on by an electrophotographic printer. When continuous paper is printed on using an electrophotographic process, the continuous paper may shrink widthwise due to the heat used to fuse the toner image formed on a photoconductive element to the continuous paper. Because the distance between the sprocket holes formed on one side of the continuous paper and the sprocket holes formed on the other side changes when the continuous paper shrinks widthwise, the sprocket holes of the continuous paper sometimes separate from the tractor pins of the first tractor or the tractor pins of the second tractor while the continuous paper is conveyed.
To prevent this, the tractor unit disclosed in JP-A-2006-8265 has a stop affixed to a position corresponding to the reference position on the support shaft, and disposes the first tractor to the reference position by pushing the first tractor towards the stop and away from the second tractor using a spring member so that the first tractor contacts the stop. When the width of the continuous paper shrinks, the first tractor moves toward the second tractor in resistance to the spring member and reduces the gap between the first tractor and the second tractor. The tractor unit can therefore prevent sprocket holes in the continuous paper from disengaging the tractor pins.
Separation of the sprocket holes from the tractor pins while the continuous paper is conveyed by a paper feed device using a tractor unit is not limited to when the width of the continuous paper shrinks as a result of the printing method. For example, when the conveyance direction of the continuous paper by the paper feed roller and the conveyance direction of the continuous paper by the tractor unit do not match precisely due to the dimensional precision of the paper feed roller and parts of the tractor unit, or the installation precision of the paper feed roller and tractor unit in the printer, the continuous paper may travel in a direction intersecting the conveyance direction due to the conveyance force of the paper feed roller, causing the sprocket holes to separate from the tractor pins. Using the tractor unit disclosed in JP-A-2006-8265 to avoid this is possible.
However, the inventor(s) has noted that when the continuous paper is set in the tractor unit disclosed in JP-A-2006-8265, the user may apply too much tension across the width of the continuous paper. More specifically, the continuous paper becomes easily skewed as it is pulled downstream by the conveyance force of the paper feed roller if there is slack across the width of the continuous paper between the first tractor and the second tractor when the continuous paper is set in the tractor unit. As a result, after engaging the tractor pins of the second tractor with the sprocket holes of the continuous paper, the user commonly pulls the second tractor in the direction away from the first tractor and applies tension across the width so that there is no slack in the continuous paper, and then clamps the second tractor to the support shaft.
If the user pulls the second tractor with such force that the first tractor moves toward the second tractor when pulling the second tractor away from the first tractor, tension will be applied to the continuous paper by both the excess tension applied by the user and the urging force of the spring member, and the second tractor will be clamped to the support shaft with excessive tension on the paper. However, the continuous paper conversely separates from the sprocket holes more easily if the continuous paper moves in a direction intersecting the conveyance direction when excessive tension is applied across the width of the continuous paper by the pair of tractors.